Ink printing apparatuses can be used for single-color or multicolor printing of a printing substrate, for example a single sheet or a web-shaped recording medium made of the most varied materials (paper, for example). The design of such ink printing apparatuses is known—see for example EP 0 788 882 B1.
Ink printing apparatuses that operate according to the Drop on Demand (DoD) principle have a print head or multiple print heads that respectively provide a plurality of printing elements. A piezoelectric printing element thereby comprises a piezoactivator that is arranged at an ink channel that is connected with a nozzle. Controlled by control voltages from a printer controller, the activators excite ink droplets in the direction of the printing substrate, which ink droplets are directed onto the printing substrate in order to apply print dots there for a print image. These control voltages are derived from the image to be printed (the print data).
In an ink printing apparatus, the ink that is used is adapted in terms of its physical/chemical composition to the print head; for example the ink is adapted with regard to its viscosity. Given low print utilization, in the printing process not all printing elements of the print head are activated; many printing elements have downtimes, with the consequence that the ink in the ink channel of these printing elements is not moved. Due to the effect of the evaporation from the nozzle opening, the danger exists that the viscosity of the ink then changes. This has the result that the ink in the printing element can no longer move optimally and exit from the nozzle. In extreme cases, the ink in the printing element dries completely and then clogs its nozzle, such that a printing with this printing element is no longer possible.
A drying of the ink in the printing elements of a print head in its downtimes represents a problem that can be prevented in that a flushing medium (for example ink or cleaning fluid) is flushed through all nozzles of the print head within a predetermined cycle. This flushing cycle can be set corresponding to the print utilization.
Furthermore, from DE 697 36 991 T2 (EP 0 788 882 B1) it is known to remedy the difficulties in the ejection of ink droplets that are caused by the change of the viscosity of the ink in the printing elements in that the piezoelectric activators of the printing elements are respectively set into vibration oscillations (also called prefire or meniscus oscillations) before or after a printing process, such that no ink droplets are ejected but the ink meniscus that projects out of the nozzle is moved so that the ink in the printing elements is thoroughly mixed. For this, control voltages are applied to the activators of the printing elements, the control voltages having a smaller amplitude and shape in comparison to the control voltages generating a print dot. It can thereby be achieved that the ink situated at the nozzle openings mixes with the ink located inside the printing elements so that the ink droplets can again be generated under approximately normal conditions in printing operation.